1. Field of the Invention
The present invention relates to an image-reject antenna apparatus having a function for intercepting an image frequency signal among signals received from an antenna unit.
2. Description of the Related Art
Conventionally, an analog unit of a receiver for wireless communications includes an antenna and a function for converting an electrical signal that is received from the antenna into a low frequency signal or a baseband signal centered at direct current (DC). In a structure in which an image frequency exists, i.e., a heterodyne system, among structures of receivers, a function for suppressing a noise signal in an image frequency band is an important factor that determines reception performance of the receiver. Methods for suppressing an image include using an image-reject filter, an image-reject mixer, or a weaver. These methods have been extensively studied.
FIG. 1 is a block diagram of a conventional heterodyne receiver, which is a representative example of a receiver having an image frequency component in a received signal according to the prior art. The conventional heterodyne receiver illustrated in FIG. 1 includes an antenna 10, an impedance matching circuit 12, and a receiver 14 including an image reject filter 16 and a down conversion mixer 18.
FIGS. 2A–2C illustrate a frequency region spectrum of a radio frequency (RF) input signal of an image-reject filter and an output signal of a down conversion mixer when a desired signal and an image signal, illustrated in FIG. 2A, pass through the image-reject filter and the down conversion mixer in the conventional heterodyne receiver of FIG. 1. FIG. 2C illustrates the output signal in a frequency domain. In this case, it was assumed that a large interference signal exists in an image frequency band among signals that are received from an antenna and that the large interference signal passes through a matching circuit.
Referring to FIGS. 1 and 2, an RF signal having a desired component and an image component is received via the antenna 10, and passes through a radio frequency (RF) band pass filter (BPF) having a central frequency of RF, which is included in the image reject filter 16, in order to pass the RF signal. The image reject filter 16 rejects the image signal from the RF signal. The rejected image signal is mixed with a local oscillation signal from a local oscillator LO, to be down-converted into an intermediate signal. An intermediate frequency (IF) BPF passes only the intermediate frequency signal. As illustrated in FIG. 2C, at an intermediate frequency, there remains a desired signal and an image signal whose amplitude is attenuated but not completely rejected by the image-reject filter.
Antennas have different characteristics depending on frequencies in view of electrical characteristics. According to a design principle of a traditional antenna and matching circuit, an antenna is designed to effectively convert an over-the-air broadcast signal in a desired frequency band into an electrical signal. In addition, a corresponding matching circuit is designed to convert an input impedance of the antenna at a desired frequency, into a reference impedance (usually, 50 Ω) without losses. That is, a frequency of interest in designing both the antenna and the matching circuit is limited to a desired band. Characteristics of the antenna and the matching circuit at a frequency other than the desired band, such as an image frequency band of a noise signal, is not considered.
Thus, a method that effectively receives a signal having a desired frequency band and suppresses an image noise signal in an antenna and a matching circuit for matching the antenna with a stipulated impedance line is not conventionally known.